Sesquialteral \Ses`qui*al"ter*al\, Sesquialterate \Ses`qui*al"ter*ate\, a. [L. sesquialter once and a half; sesqui- + alter other: cf. F. sesquialt['e]re.]

Once and a half times as great as another; having the ratio of one and a half to one.

Sesquialteral ratio (Math.), the ratio of one and a half to one; thus, 9 and 6 are in a sesquialteral ratio.

Source: Webster's Revised Unabridged Dictionary (1913)